In normal microwave transparent containers, the microwave energy can enter through the top, bottom and sides of the container. This is similar to the situation encountered during conventional oven cooking (or heating). With normal microwave foil containers the microwave energy can only enter through the top (food surface).
Prepared foods are commonly reheated in a cooking utensil on a stove top. One characteristic of this type of reheating is that the heat enters the food through the bottom of the container/utensil.
Heating the food from the bottom offers some advantages, as a result of the heat transfer mechanisms that take place. The food in contact with the base heats and becomes less dense. This provides a driving force for convective transport, the warm food rising and being replaced by cooler food from nearer the surface. The extent of this convection depends on the viscosity of the food. At a later stage of heating, bubbles of steam nucleate at or near the base and rise through the food. This transfers heat throughout the food as well as agitating the product.